The purpose of this project is to investigate the distribution and nature of the reticulo-stimulating property possessed by vaccines made from some strains of anaerobic coryneform bacteria. In an examination of almost 120 strains of anaerobic coryneforms and other organisms, it has been found that strains of P. acnes serological type II are more likely to possess high reticulo-stimulating activity than are other strains, while strains of P. granulosum are of low activity. Strains labelled Corynebacterium parvum can almost all be identified as belonging to one or the other type of P. acnes. The reticulo-stimulating material appears to be rather resistant to physical and chemical agents, such heat or proteolytic enzymes.